More ecoff sanity checks

Change FIX so that unused pointers that escape the UPDATE_RAW_END
sanity checks won't result in overflows.  Also sanity check the local
sym fdr isymBase and csym values.

	* ecoff.c (_bfd_ecoff_slurp_symbolic_info): Define FIX to set
	pointers into swapped internal data to NULL if count is zero.
	Sanity check local sym fdr_ptr->isymBase and fdr_ptr->csym.